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Date: Fri, 14 Jan 2000 19:56:04 +0000
To: rsgb_lf_group@blacksheep.org
From: "Rob Gill" <rob@twickenham.freeserve.co.uk>
Subject: Re: LF: 35 km band
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At 12:25 14/01/00 -0600, Tom Gruis, Ed.D. wrote:

>You might also check out "cave radio" and spelunkers (Sp.?) pages.  I
>have read articles in the past about these frequencies.  One thing that
>comes quickly to mind is direct loop to loop transmission.

The problem with loop to loop operation at these frequencies is that is 
subject to the inverse cube law of magnetic induction rather than an 
inverse square law. A few hundred metres is a useful range for a cave radio 
but the power required for greater range very quickly becomes impractical.

The "earth bi-pole" approach of John Taylor G0AKN, referred to by Andre' 
Kesteloot a few days ago, may be a better approach at these frequencies 
(ref: the exchanges regarding the AMRAD Antenna about a week ago). This 
technique is now frequently used for cave radio purposes with significantly 
greater range when one or both of the inductive loop antennas is replaced 
with an "earth-current" antenna. Cave radios generally operate at 73kHz or 
above rather than audio frequencies.

I know that John has recently experimented with some success at audio 
frequencies, and I think he monitors this mail reflector. I'm sure he would 
be happy to offer advice on antennas for the 35km band.

Rob

+------------------------------------------------------------------------+
Rob Gill, Cave Radio and Electronics Group
61 Cross Deep Gardens, Twickenham, Middlesex
TW1 4QZ, England
email: creg@bcra.org.uk
+------------------------------------------------------------------------+